1. Field of the Invention
The present invention relates to an axial flow turbine, and more particularly, to an axial flow turbine intended to improve a blade efficiency of a turbine nozzle in turbine stages, i.e. pressure stage, placed in a passage with an expanded diameter formed in an axial direction of a turbine shaft (turbine rotor) in a turbine casing.
2. Related Art
Recently, in a motor employed for a power plant, for example, a steam turbine unit or system includes stages of a high pressure turbine, an intermediate pressure turbine, and a low pressure turbine for increasing outputs. The respective pressure turbines allow heat energy of steam supplied from a steam source to have an expansion work so as to obtain a rotating power. For the purpose of improving the power generation efficiency, it is essential to find the way how the expansion work is enhanced in the respective turbine stages for obtaining the rotating power. Specifically, the high pressure turbine is expected to bear more loads to increase the steam pressure for the expansion work compared with the intermediate and low pressure turbines.
Due to the high proportion of the work supplied by the high pressure turbine to that of the entire steam turbine, the improvement of the output per high pressure turbine stage may be significant for improving the output of the entire turbine unit.
In a generally employed high pressure turbine, a plurality of turbine stages are arranged in a row for allowing the steam that flows in the axial direction of the turbine shaft to have the expansion work. The aforementioned high pressure turbine is called as an axial flow type turbine.
The turbine stage is formed by combining cascaded turbine nozzles in a circumferential direction of the turbine shaft, and turbine rotor blades corresponding to the cascaded turbine nozzles.
A nozzle cascade constituting a generally employed axial flow turbine among the turbines formed by combining the turbine nozzles and the turbine rotor blades is shown in FIG. 2. Referring to FIG. 2, a plurality of nozzle blades 10 are supported to be placed between an inner (diaphragm) ring 11 and an outer (diaphragm) ring 12 in the circumferential direction of a turbine shaft, not shown. In the high pressure turbine at a relatively low blade height, a secondary flow loss is a dominant cause to reduce the internal efficiency of the turbine. Within an annular passage of the turbine as shown in FIG. 2, a secondary vortex 16 is generated by a hydrodynamic load 15 that causes the fluid to flow from a ventral side at a high blade surface pressure to a back side at a low pressure around an inner radial wall surface 13 and an outer radial wall surface 14 of the nozzle blade 10. The secondary flow loss is considered to be caused by the secondary vortex 16. As shown in FIG. 3 that represents an energy loss distribution in the direction of the height of the nozzle blade 10, high energy loss areas generally distribute around the inner and the outer radial wall surfaces 13 and 14, respectively. Further, since the height direction range of the area hardly changes irrespective of the increase in the blade height, degradation of the efficiency owing to the secondary flow loss is reduced as the blade height increases.
A turbine nozzle having the nozzle blade 10 curved toward an outlet side (which is hereinafter referred to as a curved nozzle) has been widely used for the purpose of reducing the secondary flow loss.
FIG. 4 shows a configuration of a generally employed curved nozzle. One of reference values for defining the curved configuration is represented by a curvature range in the blade height direction. Further, there are several methods for setting the curvature range including a typical method in which the curvature of a center of the blade height is set to a maximum value such that the nozzle blade is entirely curved over a whole range in the blade height direction, and a similarity expansion is made as the increase in the blade height. In this case, the absolute value of the curvature range changes as the blade height varies.
Meanwhile, the use of the curved nozzle may cause an adverse effect to deteriorate the nozzle blade performance at the center of its height, counteracting the improvement of the performance achieved by reducing the secondary loss. In this case, the curved configuration serves to press the fluid against the inner and outer radial wall surfaces 13 and 14 on the inner and outer rings 11 and 12 to suppress the secondary flow loss. On the other hand, the fluid flows at the reduced flow rate around the center of the nozzle blade in the height direction, which is supposed to be unaffected by the secondary loss, and accordingly exhibits the excellent performance.
FIG. 5 shows each of changes in the loss distribution of the curved nozzle and the normal nozzle with no curvature.
In the case where the blade height is at a low level, the effect by the secondary flow may be suppressed. The performance of the nozzle blade may be expected to be improved over its entire height. However, in the generally configured nozzle blade in which the curvature range increases as the increase in the blade height, the adverse effect owing to the reduced flow rate of the fluid at the center of the nozzle blade height may further be worsened. This may deteriorate the improvement of the entire performance of the curved nozzle.
Publication of PCT Japanese Translation Patent Publication No. 2002-517666 has proposed, as a method of improving the above problem, a method of forming the curved nozzle at the limited area around the inner and outer radial wall surfaces 13 and 14 on the inner and outer rings 11 and 12 with respect to the formation of a cross section of the flow passage defined by adjacent turbine nozzles.
In the disclosed method, the center of the nozzle blade height has no curvature area, which is expected to provide the effect for suppressing the performance degradation caused by the reduction in the flow rate around the center of the nozzle blade height compared with the case in which the nozzle blade is curved over the entire height. In the disclosed method, the curvature range is defined as the proportion of the blade height. The curvature range may be increased as the blade height increases, and accordingly the performance improvement is deteriorated as the flow rate at the center of the nozzle blade height reduces.
Conversely, in the case where the blade height is at the low level, the curvature range is reduced. However, as a secondary flow area in almost a constant range exists irrespective of the blade height, the effect for suppressing the secondary flow cannot be sufficiently obtained owing to insufficient curvature range.
As described above, the loss caused by the secondary vortex generated around the wall surface in a base portion and a tip portion of the turbine nozzle has been considered as the main cause for reducing the internal efficiency of the high pressure turbine at a relatively low blade height.
It is well known that the curved nozzle has been widely used for the purpose of reducing the secondary flow loss. The curvature range in the blade height direction is one of reference values that indicate the configuration, and several methods have been proposed for determining such curvature range. In one of those methods, the nozzle blade is curved over its entire height so as to make a similarity expansion as the increase in the blade height.
With the thus configured curved nozzle, the fluid is pressed against the wall surface around the upper and lower wall surfaces to suppress the secondary flow loss. However, the flow rate of the fluid is reduced at the center of the blade height, thus degrading the excellent performance of the center area which has not been affected by the secondary flow, thus deteriorating improvement of the entire performance.
In the general method where the absolute value in the curvature range changes in accordance with the blade height even if the range influenced by the secondary flow loss hardly changes irrespective of the blade height, the flow rate distribution at the outlet of the turbine nozzle is found disproportionately at the area especially around the wall surface of the inner and the outer rings 11 and 12 as the blade height increases. This may further worsen the adverse effect to the curved nozzle as described above.
The above-described PCT Japanese Translation Patent Publication No. 2002-517666 discloses a method of curving the configuration of the passage defined by the adjacent turbine nozzles only at the portion around the upper and lower wall surfaces on the inner and the outer rings 11 and 12 for solving the aforementioned problem. It is considered that the use of the configuration limiting the curvature range to the portion around the upper and lower wall surfaces on the inner and outer rings 11 and 12 in the blade height direction may suppress the decrease in the flow rate of the fluid at the center of the blade height while suppressing the secondary flow loss. The disadvantage of the nozzle blade curved over the entire height, thus, may be compensated. In this method, the curvature range is defined as the proportion of the blade height.
In the case where the blade height is at the high level, the curvature range is expanded. This may fail to completely eliminate the adverse effect caused by the decrease in the flow rate of the fluid at the center of the blade height. In the case where the blade height is at the low level, the curvature range is reduced. In this case, the effect for suppressing the secondary loss cannot be sufficiently obtained owing to insufficient curvature range because the area influenced by the secondary loss is ranged at a height that is almost kept constant.